Precipitated calcium carbonate (PCC) is manufactured on a commercial scale for use in a variety of industrial, cosmetic and pharmaceutical products. Generally, precipitated calcium carbonate is made by heating crushed limestone at high temperatures, breaking the raw calcium carbonate into lime (CaO) and carbon dioxide gas (CO2). Addition of the water to the lime (a process called “slaking”) yields Ca(OH2). The slaked lime (or “milk of lime”) is treated with carbon dioxide gas. The resulting calcium carbonate precipitates from the aqueous solution, yielding a slurry of precipitated calcium carbonate that can be neutralized, milled, filtered, dewatered and/or dried, as desired.
Slurries of precipitated calcium carbonate are susceptible to microorganisms. To deter microorganisms, a preservative such an aldehyde can be added to the slurry. For example, U.S. Patent Application Publication No. 2009/0088483 discloses combinations of a dialdehyde (such as glutaraldehyde) and a formaldehyde-releasing agent such as (ethylenedioxy)dimethanol. Unfortunately, these preservatives have their limitations. Glutaraldehyde, for example, is unstable at alkaline pH, and is therefore ineffective as a long-term preservative. Furthermore some bacterial strains metabolize formaldehyde (see, for example, Di Maiuta et al. (2009) International Biodeterioration & Biodegradation 63:769-777), permitting bacterial growth even in a treated PCC slurry.
There is a need for improved preservation methods for precipitated calcium carbonate compositions.